Designers, manufacturers, and users of electronic computers and computing systems require reliable and efficient equipment for storage and retrieval of information in digital form. Conventional storage systems, such as magnetic disk drives, are typically utilized for this purpose and are well known in the art. However, the amount of information that is digitally stored continually increases, and designers and manufacturers of magnetic recording media work to increase the storage capacity of magnetic disks.
In conventional magnetic disk data/information storage, the data/information is stored in a continuous magnetic thin film overlying a substantially rigid, non-magnetic disk. Each bit of data/information is stored by magnetizing a small area of the thin magnetic film using a magnetic transducer (write head) that provides a sufficiently strong magnetic field to effect a selected alignment of the small area (magnetic grain) of the film. The magnetic moment, area, and location of the small area comprise a bit of binary information which must be precisely defined in order to allow a magnetic read head to retrieve the stored data/information.
Such conventional magnetic disk storage media incur several drawbacks and disadvantages which adversely affect realization of high areal density data/information storage, as follows:
(1) there is an infinite number of possibilities for the magnetic moments of the continuous magnetic film, and as a consequence, the write head must be able to write very precisely in order to precisely define, without error, the magnetic moment, location, and area of each bit on the magnetic film;
(2) since the continuous film tends to link exchange and magnetostatic interaction between neighboring magnetic bits, when the bits are very close, writing of one bit can result in writing of neighboring bits because of the exchange and magnetostatic interaction, causing errors in reading;
(3) the absence of physical boundaries between many bits of the continuous magnetic film cause the writing and reading process to occur in a “blind” fashion, i.e., the location of each bit is determined by calculating the movements of the disk and the read or write heads instead of physically sensing the actual bit location;
(4) the boundaries between adjacent pairs of bits tend to be ragged in continuous magnetic films, resulting in noise generation during reading; and
(5) the requirement for increased areal recording density has necessitated a corresponding decrease in recording bit size or area. Consequently, recording bit sizes of continuous film media have become extremely minute, e.g., on the order of nanometers (nm). In order to obtain a sufficient output signal from such minute bits, the saturation magnetization (Ms) and thickness of the film must be as large as possible. However, the magnetization quantity of such minute bits is extremely small, resulting in a loss of stored information due to magnetization reversal by “thermal fluctuation”, also known as the “superparamagnetic effect”.
Regarding item (5) above, it is further noted that for longitudinal type continuous magnetic media, wherein the magnetic easy axis is oriented parallel to the film plane (i.e., surface), magnetization reversal by the superparamagnetic effect may occur even with relatively large magnetic particles or grains, thereby limiting increase in areal recording density to levels necessitated by current and future computer-related applications. On the other hand, for perpendicular type continuous magnetic media, wherein the magnetic easy axis is oriented perpendicular to the film plane (i.e., surface), growth of the magnetic particles or grains in the film thickness direction increases the volume of magnetization of the particles or grains while maintaining a small cross-sectional area (as measured in the film plane). As a consequence, onset of the superparamagnetic effect can be suppressed for very small particles or grains of minute width. However, further decrease in grain width in perpendicular media necessitated by increasing requirements for areal recording density will inevitably result in onset of the superparamagnetic effect even for such type media.
The superparamagnetic effect is a major limiting factor in increasing the areal recording density of continuous film magnetic recording media. Superparamagnetism results from thermal excitations which perturb the magnetization of grains in a ferromagnetic material, resulting in unstable magnetization. As the grain size of magnetic media is reduced to achieve higher areal recording density, the superparamagnetic instabilities become more problematic. The superparamagnetic effect is most evident when the grain volume V is sufficiently small such that the inequality KμV/kBT>40 cannot be maintained, where Kμ is the magnetic crystalline anisotropy energy density of the material, kB is Boltzmann's constant, and T is the absolute temperature. When this inequality is not satisfied, thermal energy demagnetizes the individual magnetic grains and the stored data bits are no longer stable. Consequently, as the magnetic grain size is decreased in order to increase the areal recording density, a threshold is reached for a given Kμ and temperature T such that stable data storage is no longer possible.
So-called “patterned” or “bit patterned” magnetic media have been proposed as a means for overcoming the above-described problem of conventional continuous magnetic media associated with magnetization reversal via the superparamagnetic effect, e.g., as disclosed in U.S. Pat. No. 5,956,216, the entire disclosure of which is incorporated herein by reference. The term “patterned” media generally refers to magnetic data/information storage and retrieval media wherein a plurality of discrete, independent regions of magnetic material form discrete, independent magnetic elements which function as recording bits are formed on a non-magnetic substrate. Since the regions of ferromagnetic material comprising the magnetic bits or elements are independent of each other, mutual interference between neighboring bits can be minimized. As a consequence, patterned magnetic media are advantageous vis-à-vis continuous magnetic media in reducing recording losses and noises arising from neighboring magnetic bits. In addition, patterning of the magnetic layer advantageously increases resistance to domain wall movement, i.e., enhances domain wall pinning, resulting in improved magnetic performance characteristics.
Generally, each magnetic bit or element has the same size and shape, and is composed of the same magnetic material as the other elements. The elements are arranged in a regular pattern over the substrate surface, with each element having a small size and desired magnetic anisotropy, so that, in the absence of an externally applied magnetic field, the magnetic moments of each discrete magnetic element will aligned along the same magnetic easy axis. Stated differently, the magnetic moment of each discrete magnetic element has only two states: the same in magnitude but aligned in opposite directions. Each discrete magnetic element forms a single magnetic domain or bit and the size, area, and location of each domain is determined during the fabrication process.
During writing operation of such patterned media, the direction of the magnetic moment of the single magnetic domain element or bit is flipped along the easy axis, and during reading operation, the direction of the single magnetic domain element or bit is sensed. The direction of the magnetic easy axis of each single magnetic domain, element, or bit can be parallel or perpendicular to the surface of the domain, element, or bit, corresponding to conventional continuous longitudinal and perpendicular media, respectively. Stated differently, the nature (i.e., type) of the magnetic recording layer of the magnetic domain elements or bits is not critical in patterned media, and may, for example, be selected from among longitudinal, perpendicular, laminated, anti-ferromagnetically coupled (AFC), granular, superlattice, etc., types.
Patterned media in disk form offer a number of advantages relative to conventional disk media. Specifically, the writing process is greatly simplified, resulting in much lower noise and lower error rate, thereby allowing much higher areal recording density. In patterned disk media, the writing process does not define the location, shape, and magnetization value of a bit, but merely flips the magnetization orientation of a patterned single domain magnetic structure. Writing of data can be essentially perfect, even when the transducer head deviates slightly from the intended bit location and partially overlaps neighboring bits, as long as only the magnetization direction of the intended bit is flipped. By contrast, in conventional magnetic disk media, the writing process must define the location, shape, and magnetization of a bit. Therefore, with such conventional disk media, if the transducer head deviates from the intended location, the head will write to part of the intended bit and to part of the neighboring bits. Another advantage of patterned media is that crosstalk between neighboring bits is reduced relative to conventional media, whereby areal recording density is increased. Each individual magnetic element, domain, or bit of a patterned medium can be tracked individually, and reading is less jittery than in conventional disks.
In bit patterned media, interaction between the discrete magnetic elements is purely magnetostatic, and is inversely proportional to the cube of the distance, r, between neighboring magnetic elements (bits), i.e., 1/r3. FIG. 1 illustrates the situation when the discrete elements are in the form of circularly shaped columns arranged in a hexagonal close-packed array, i.e., where each magnetic element is surrounded by 6 first nearest neighbor magnetic elements, where r=a; 6 second nearest neighbor magnetic elements, where r=a√3; and so on. On average, the number of magnetic elements or bits that surround a magnetic element or bit increases with the square of the distance r, i.e., as r2. As a consequence, the total magnetostatic interaction between a magnetic element and its neighboring magnetic elements decreases with distance r as 1/r (noting that the magnetostatic interaction between two magnetic elements decreases with distance r as 1/r3 and the number of magnetic elements increases with distance r as r2). It follows, therefore, that the coercivity field, Hc, of each magnetic element depends upon the magnetic state of the neighboring magnetic elements due to the magnetostatic interaction. A drawback of conventionally structured bit patterned magnetic media arising from the dependence of the coercivity field Hc of each magnetic element on the magnetic state of the neighboring magnetic elements due to the magnetostatic interaction is disadvantageous creation of a distribution of Hc, resulting in a deterioration of the magnetic performance characteristics of such bit patterned media. In addition, the magnetostatic interaction disadvantageously lowers the thermal stability of the magnetic elements.
In view of the foregoing disadvantages and drawbacks resulting from magnetostatic interaction between neighboring magnetic elements of conventionally structured bit patterned media, leading to a distribution of Hc with attendant deterioration of magnetic properties and reduction in thermal stability, there exists a clear need for improved bit patterned media (and methodology therefor) which function in optimal fashion such that the above-described disadvantages and drawbacks are effectively eliminated or at least minimized.